The invention concerns a method of amplitude equalization of a plurality of optical signals having mutually different wavelengths, said optical signals being launched into a first end of an optical fibre, and a corresponding apparatus.
Optical line amplifiers are increasingly used in long optical transmission cables instead of traditional electrical regenerators. One of the advantages of the optical amplifiers is that they can be used in wavelength division multiplexed (WDM) systems to simultaneously amplify a large number of individual transmission signals having different wavelengths within the gain band of the optical amplifier, which will typically be an erbium doped fibre amplifier (EDFA).
For wavelength division multiplexed applications, however, a serious problem is that erbium doped fibre amplifiers in general have a certain gain variation over the gain band, which means that the various wavelengths will not experience quite the same gain. Of course, this problem is aggravated when several amplifiers are coupled in cascade, and, therefore, this limits the number of optical amplifiers which can be cascaded, as the gain variation over the gain band will practically be the same for all the amplifiers in the cascade.
The gain variation may e.g. have as a result that, at the receiver end of an optical cable, the signal level of the signal or signals subjected to the lowest gain en route, will be below the detection or sensitivity limit of the optical receiver arranged there, so that the signal cannot not be received. It may also occur that the signal level of the signal or signals subjected to the highest gain en route, will exceed the non-linearity limit of the system, in which case severe signal distortions will appear. Possibly, the amplifiers en route may be adapted to reduce their gain to a level below the specified one; but, the effect of this is just that low gain signals are reduced additionally. Finally, when the signal levels are different, crosstalk from the strong to the weak signals may increase, so that the quality of the weak signals may degrade to an unacceptable degree.
It is known to counteract this by performing equalization on e.g. the output of an optical amplifier.
European Patent Application EP 543 314 teaches a solution wherein equalization is performed by passing the optical signal having the different wavelengths, which may have been subjected to disuniform gain, through a polarization independent acoustically tuned optical filter (PIATOF) Immediately before or after this filter, a small part of the optical signal is tapped for a demultiplexer, in which the individual wavelengths are separated and converted into corresponding electrical signals. Then, on the basis of these electrical signals, a control circuit determines a plurality of coefficients associated with the respective wavelengths. The coefficients are fed to the polarization independent acoustically tuned optical filter, which can then attenuate each individual wavelength depending on the coefficient associated with the wavelength concerned.
However, this solution has the serious drawback that the optical signals must be converted into electrical signals, and that the entire signal processing then takes place purely electrically. The electrical control circuit for the polarization independent acoustically tuned optical filter is rather complex and thus expensive to implement, and the conversion into electrical signals moreover results in increased inaccuracy. One of the advantages of optical fibre amplifiers is precisely that they operate purely optically, and this advantage will therefore be lost if, nonetheless, the optical signals have to be converted into electrical signal owing to the equalization.
Another solution is known from European Patent Application EP 685 946 describing an optical amplifier having a circulator with three ports. Two of the ports constitute the input and the output, respectively, of the amplifier, while the third port is connected to an optically amplifying fibre. Spectrally selective Bragg grating reflectors are formed at specific intervals along the fibre. The intervals between the reflectors are chosen such that each wavelength is reflected back to the circulator at a distance in inverse proportion to the gain per unit length of the wavelength concerned in the fibre. As a result, all wavelengths are amplified by the amplifier to the same extent. While this solution can operate purely optically, it is a static solution, as the individual reflectors must be placed in the fibre in advance on the basis of the knowledge of the amplifier gain at the individual wavelengths. Thus, it is not possible to allow for gain variations because of fibre parameter tolerances and especially not the dynamic variations which will always occur in practice in such an amplifier, i.a. because of saturation.
Accordingly, an object of the invention is to provide a method and an optical amplifier of the type stated in the opening paragraph, which are capable of operating purely optically, while being capable of performing dynamic equalization of the individual channels in a wavelength division multiplexed system.
This is achieved according to the invention by a method, wherein the optical signals, before being launched into the fibre, are amplified to a level at which at least one of the signals exceeds a stimulated Brillouin scattering (SBS) threshold value characteristic of the fibre, so that part of the signal energy in the fibre is transferred to a Stokes signal propagating in the fibre in a direction opposite to said optical signals.
The SBS effect is known per se, but is normally considered as a deleterious effect because it limits the maximal optical power that can be transmitted through an optical fibre.
However, applications utilizing the SBS effect to achieve desired effects are also known. As an example, DE 40 16 331 mentions a Brillouin fibre amplifier utilizing the effect for selective amplification of a single optical frequency. Also in EP 261 876 the SBS effect is used to amplify a selected optical signal from a plurality of optical signals thereby providing narrowband reception of the selected signal. This effect is so to say opposite to equalization. In U.S. Pat. No. 4,977,620 the SBS effect is used to amplify the carrier component of a coherent modulated optical wave received in an optical homodyne detection system. U.S. Pat. No. 5,515,192 mentions an optical signal generator that can be amplitude modulated between levels which are above and below the SBS threshold value, respectively, so that the SBS effect only occurs when the signal exceeds the threshold value. The SBS effect, however, is only utilized with a single optical frequency in these known applications. Systems with several optical frequencies are not mentioned in relation to the SBS effect.
When at any rate one of the optical wavelengths exceeds the SBS threshold value, this will be attenuated in the fibre owing to the SBS effect, as the signal will substantially be reduced to the threshold value at the wavelength or wavelengths exceeding the threshold value. The reason is that the SBS effect is sufficiently narrow-banded for it to occur for each individual wavelength independently of the signals at other wavelengths. The excess part of the signal will be converted into the oppositely directed Stokes wave. The wavelengths whose amplitude does not exceed the threshold value, will pass the fibre with just a quick slight attenuation or no attenuation at all. This ensures that the amplitude of the strongest signal or signals is attenuated with respect to the others, so that amplitude equalization takes place.
In an embodiment, which is defined in claim 3, said Stoke signal or signals are tapped from the fibre at its first end and are fed back to the second end of the fibre, thereby generating an SBS laser. This reduces the SBS threshold value, and it may moreover be regulated in response to the SBS laser cavity parameters, which are determined by the fibre type selected. The reason is that the Stokes signal is increased considerably owing to the feedback, which stabilizes the SBS process. The use of an SBS laser moreover permits a shorter interaction length of the fibre in which the SBS process takes place, which will reduce the effect of other non-linear phenomena, such as FWM (Four Wave Mixing) or SPM (Self Phase Modulation).
In a preferred embodiment, which is defined in claim 4, said Stokes signals are amplified in the feedback from the first end of the fibre to its second end in an optical amplifier. This results in a further stabilization of the SBS effect and a further reduction in the threshold value. One reason is that the optical amplifier may be used for generating noise which can stimulate and average the SBS process.
As the information content in said optical transmission systems is usually transferred by modulating each of said optical wavelengths, it may occur that each channel has a bandwidth which exceeds the SBS bandwidth, since, as mentioned before, the SBS effect is very narrow-banded. In that case, only the part of the channel spectrum within the SBS bandwidth will be attenuated, which means that the signal will be distorted. This may be counteracted by varying each of said mutually different wavelengths, as stated in claim 5, periodically about a nominal wavelength. Since the SBS effect has a certain inherent sluggishness, the SBS bandwidth of each individual wavelength will hereby be increased so that at any rate most of the channel spectrum will be within the SBS bandwidth, thereby obviating or reducing the distortion.
As mentioned, the invention also concerns an apparatus for amplitude equalization of a plurality of optical signals having mutually different wavelengths. The apparatus comprises an optical fibre having an input end where said optical signals may be supplied to the fibre, and an output end where said optical signals may be tapped from the fibre after equalization.
When the fibre has a characteristic stimulated Brillouin scattering (SBS) threshold value which corresponds to a preselected maximum amplitude value of the equalized optical signals on the output end of the fibre, it is ensured that optical signals exceeding the SBS threshold value will be attenuated in the fibre owing to the SBS effect, as the signal at the wavelength or wavelengths exceeding the threshold value will be substantially reduced to the threshold value. The reason is that the SBS effect is sufficiently narrow-banded for it to occur for each individual wavelength independently of the signals at the other wavelengths. The excess part of the signal will be converted into an oppositely directed Stokes wave. The wavelengths whose amplitude does not exceed the threshold value, will pass the fibre with a quite slight attenuation or no attenuation at all. This ensures that the amplitude of the strongest signal or signals is attenuated with respect to the others, so that amplitude equalization takes place.
Further, when the optical signals at the input end of the fibre are caused to have a level at which all these signals exceed said threshold value, it is ensured that all the signals are attenuated to the same level, which in turn substantially corresponds to the SBS threshold value, so that complete equalization takes place.
Further, when, as stated in claim 7, the apparatus comprises means for feedback of optical signals from said input end to said output end, said Stokes signals may be fed back, thereby generating an SBS laser. This reduces the SBS threshold value, which will simultaneously be more stable and may be regulated depending on the SBS laser cavity parameters, which are determined by the fibre type selected. The reason is that the Stokes signal is increased considerably owing to the feedback, which stabilizes the SBS process.
In a preferred embodiment, which is defined in claim 8, said feedback means comprise an optical amplifier. This results in a further stabilization of the SBS effect and a further reduction in the threshold value. One reason is that the optical amplifier may be used for generating noise which can stimulate and average the SBS process. The optical amplifier may expediently be an erbium doped fibre amplifier, as stated in claim 9.